The bodies of many older Americans are practically bionic: more than 770,000 hip and knee replacements are performed each year in the United States.

Now another aging joint is fast becoming a candidate for replacement. This year, 4,400 patients are expected to undergo surgery to replace arthritic or injured ankles with artificial joints made of metal alloys and lightweight plastic, according to industry estimates.

Four models are commonly used in the United States, with Food and Drug Administration approval. And demand is expected to grow as more and more baby boomers hobble into their 60s and 70s with debilitating ankle pain.